Dr Fleming's Remarks on the Defoliation of Trees. 73 



Trees, in reference to the duration of their leaves, appear 

 capable of division into three classes. In the first class may- 

 be included those in which the leaves cease to exercise their 

 functions whenever the bud has been perfected. In the se- 

 cond, the leaves continue to exercise their functions until new 

 ones are produced in the following season. In the third class, 

 the leaves continue to exercise their functions for several 

 years. 



In trees of the first class the leaf may, with propriety, be 

 termed " Folium deciduum^ Its principal function appears 

 to be connected with the ripening of the bud, and, when this 

 object is accomplished, the leaf changes colour and dies. The 

 falling of such leaves takes place, as, indeed, in all other 

 cases, in the order in which they were evolved. Hence by 

 midsummer we witness, in willows, for example, a considerable 

 portion of the lower part of the shoot naked, its leaves havino- 

 fallen, while towards the extremity its foliage is fresh and in- 

 creasing in quantity. 



In those trees in which two evolutions of buds take place 

 in the course of the season, as in the beech, the leaves formed 

 on the spring shoot change their colour and die sooner than 

 those evolved at a later period on the summer shoot. 



In those trees which seem to lose all their leaves suddenly, 

 as the ash, the same order of defoliation actually prevails. 

 The first evolved leaves of spring have perished, by degrees, 

 in the course of the summer. Those with which the tree is 

 clothed at the end of the season, are connected with the termi- 

 nal buds, which, by becoming perfect about the same timc > 

 permit the leaves connected with them to fall off in rapid suc- 

 cession. 



The leaves, in some cases, after they have ceased to exer- 

 cise their functions, continue attached until the following 

 spring, as the beech in hedges. When this plant, however, 

 is not under the influence of the shears, the leaves fall oh" in 

 the usual manner. The cause of this want of resemblance be- 

 tween individuals of the same species, may be found in an ex- 

 amination of the different circumstances in which they are 

 placed. When the beech grows exposed to free air and 

 sunshine, the buds attain their true size, the leaves execute 



